1. Field of the Invention
The present invention relates to a metallic colloidal solution and an inkjet-use metallic ink both suitable for forming a fine wiring circuit, a thin conductive film with a uniform thickness, and the like.
2. Description of the Background Art
Wiring circuits, conductive films, and the like are formed by using extremely minute metallic particles having a diameter of several nanometers to tens of nanometers or so. More specifically, such metallic particles are dispersed in a dispersion medium under the presence of a dispersant for preventing aggregation and increasing the dispersibility to obtain a metallic colloidal solution. The metallic colloidal solution is used as an ink for various printing methods and ink-applying methods. The ink is printed or applied on a substrate and, as required, is baked to form a wiring circuit or a conductive film.
As the metallic colloidal solution, a water-based solution using water as the dispersion medium has been widely used. In addition, the solution's properties such as viscosity, surface tension, and vapor pressure (a boiling point) are limited within a narrow range. However, wiring circuits and conductive films have been formed in recent years through various printing methods and ink-applying methods, such as the spin coating method, the screen printing method, and the dispenser application method. Consequently, the metallic colloidal solution is required to have properties suitable for individual printing methods and ink-applying methods.
Furthermore, as the forming method of a wiring circuit and a conductive film, the inkjet printing method, which uses an inkjet printer, has been attracting attention in recent years. Studies are being made to use the metallic colloidal solution as the metallic ink for the inkjet printing method. However, the conventional metallic colloidal solution is usually produced as a water-based solution using water as the dispersion medium. In addition, the solution's properties such as viscosity, surface tension, and vapor pressure (a boiling point) are limited within a narrow range.
In view of the above-described circumstances, studies are being conducted to use not only water but also various organic solvents as the dispersion medium, which directly affects the properties of the metallic colloidal solution. For example, the published Japanese patent application Tokukaihei 11-80647 has disclosed the below described methods of producing a metallic colloidal solution (see claims 11 and 12 and sections 0042, 0045, and 0046 of this application). This application utilizes the liquid-phase reduction method, which is known as a method capable of producing metallic particles with a uniform particle diameter. More specifically, Tokukaihei 11-80647 applies a method of producing a water-based metallic colloidal solution by depositing metallic particles through the reduction of metallic ions in water under the presence of a dispersant. Tokukaihei 11-80647 has disclosed the following methods:    (a) a method of producing a metallic colloidal solution whose dispersion medium is an organic solvent. In this method, first, a metallic compound, which is the source of metallic ions, is solved in an organic solvent. After the addition of a dispersant, the metallic ions are reduced to deposit metallic particles.    (b) a method of producing a metallic colloidal solution whose dispersion medium is a mixed solvent of water and a water-soluble organic solvent. In this method, first, a metallic compound, which is the source of metallic ions, is solved in water. After the addition of a water-soluble organic solvent and a dispersant, the metallic ions are reduced to deposit metallic particles.
Another published Japanese patent application, Tokukai 2001-35255, has disclosed a method of producing a metallic colloidal solution in which metallic particles are dispersed in an organic solvent (in Tokukai 2001-35255, the solution is called a liquid in which extremely fine silver particles are dispersed independently) (see section 0006 of Tokukai 2001-35255). This application utilizes a method of producing metallic particles through the vapor-phase growth method. First, metallic particles growing in a vapor phase are made contact with a vapor of a high-boiling-point organic solvent such as mineral spirits. Then, the metallic particles are cooled and recovered to produce the solution.
However, in one of the methods disclosed in Tokukaihei 11-80647, which one uses an organic solvent as the dispersion medium, the types of metallic compound and reducing agent both having good solubility in a specific organic solvent are limited. This limitation poses a problem of limiting the type of metallic particles to be formed.
On the other hand, in the other method, which uses a mixed solvent of water and a water-soluble organic solvent, although commonly used water-soluble metallic compounds and water-soluble reducing agents can be used, many of them have low solubility in a water-soluble organic solvent. As a result, at the time the water-soluble organic solvent is added, some of them are deposited or forced to behave adversely to cause the reaction system to become prone to produce nonuniformity in concentration.
This nonuniformity in concentration causes the formed metallic particles to have variations in particle diameter. Consequently, the produced wiring circuit and conductive film may become nonuniform in properties such as structure and conductivity.
When this metallic colloidal solution is used as an inkjet-use metallic ink, the nozzle and another small opening of an inkjet printer tend to be clogged. Furthermore, the formed wiring circuit and conductive film may become non-uniform in properties such as structure and conductivity. In addition, unreacted metallic compounds may enter the metallic particles as an impurity. When this occurs, the metallic particles and, consequently, the wiring circuit and conductive film may be impaired in properties such as conductivity.
On the other hand, the method disclosed in Tokukai 2001-35255 is limited to the application that uses a high-boiling-point organic solvent, which requires careful handling with regard to safety and environment. In other words, this method has a drawback of a narrow range of application. Furthermore, when this metallic colloidal solution is used as an inkjet-use metallic ink, the organic solvent such as mineral spirits may dissolve the adhesive used in the head and other components of the inkjet printer.